Much like in in-addr.arpa, the reverse DNS zone for IPv4 addresses,
the addresses here are written "backwards", because of the need
for the structure of DNS information correspond with the ownership
of the actual address delegations.

The ip6.int zone is described in RFC 1886 (DNS Extensions to support IP version 6).
According to RFC 2874 (DNS Extensions to Support IPv6 Address Aggregation and Renumbering), it's supposed to be replaced by the ip6.arpa zone, wherein
the address space delegations are to be made using few new DNS features,
notably DNAME records and "bit-stringlabels" (BITLBLs).
There has been some discussion about the ip6.arpa zone on IPv6-related mailing lists,
but so far the current situation is that all resolvers use the ip6.int zone.